1. Field of the Invention
This invention relates to high strength low carbon steel wire rods having excellent cold drawing properties and to a method of producing them. This invention further relates to a method of producing ultra-fine steel wires using the high strength low carbon steel wire rods and also to brass-plated ultra-fine steel wires.
2. Description of the Prior Art
Steel wires drawn from steel wire rods into diameters from several millimeters to several tens of micrometers have been used, depending on their diameters, in various applications such as PC wires, various kinds of spring wires, rope wires, tire bead wires, tire cord wires, high pressure hose wires, switching wires, corona wires and dot printer wires. Since ultra-fine steel wires are usually produced from rolled wire rods of high carbon steel of about 5.5 mm diameter by several cold drawing steps during each of which steps reduction in the toughness of drawn wire rods is prevented by the application of a patented treatment several times during the course of production, a number of production steps are required and accordingly the production costs inevitably increase.
On the other hand, it is also possible to draw ultra-fine wires by intense work from steel wire rods made of pure iron or low carbon ferrite-pearlite steels, but the strength of the ultra-fine wire products is low since their strength is diminished by the drawing operation. That is, even in the drawn wires subjected to intense work at a rate of 95-99%, the strength of the drawn wires is only from 70 to 130 kgf/mm.sup.2 and strengths greater than 170 kgf/mm.sup.2 cannot be attained. Further, even at drawing at a rate greater than 99%, the strength is still lower than 190 kgf/mm.sup.2.
Wire rods having a tempered martensite structure prepared by the heat treatments of hardening and tempering are also known. However, since no wire rods having the desired workability can be obtained by hardening of the rods, workability has only been obtained by significantly reducing the strength of the wire rods by a tempering treatment and, accordingly, strong and ductile steel wires cannot be obtained. Moreover, hardened wire rods suffer from surface cracking during the pickling step which is applied as a treatment prior to the drawing step. The rods also inevitably exhibit insufficient ductility.
The present inventors have conducted intensive studies for the preparation of high strength and highly ductile steel wire rods instead of conventional ferrite-pearlite wire rods, pearlite wire rods and tempered martensite wire rods and, as a result, have found that steel wire rods having composite structures in which a fine low temperature transformation phase comprising an acicular bainite, martensite and/or mixed structure thereof that comprises predetermined chemical compositions and may partially contain retained austenite is uniformly dispersed in a ferrite phase, have excellent intense workability. The inventors have already filed a U.S. patent application based on such findings which is now U.S. Pat. No. 4,578,124. However, it has also been found that even the steel wire rods having such excellent cold drawing properties show degradation in ductility and sometimes break when drawn at a drawing speed of higher than 20 m/min. Such a degradation in ductility is a problem characteristic of composite structures in general and are not restricted only to the acicular structure, when the steel wire rods, before drawing, are subjected to quenching.
Specifically, upon high speed drawing, ductility degrades even in steel wire rods which have a metal structure which exhibit cold drawing properties because of the temperature increase during drawing work because of the high aging effect. In addition, the effect of hydrogen tends to develop when the strength of the drawn wire rod is increased by the drawing work and the tensile strength increases to greater than about 150 kgf/mm.sup.2. The effect of hydrogen is particularly significant in the case where the strength is greater than about 200 kgf/mm.sup.2.
For instance, FIG. 1 shows the tensile strength and the reduction of area at break of a drawn wire obtained from a high strength wire rod of 7.5 mm diameter having a mixed structure comprising 8% ferrite and 92% martensite prepared by rolling and then directly hardening the steel material represented by the reference R2 and having chemical compositions shown in Table 1 at a drawing speed of 1 m/min or 50 m/min. That is, a drawn wire of high strength greater than 200 kgf/mm.sup.2 and high ductility can be obtained at a working rate of 70 to 80% in the case of using a drawing speed of m/min. However, since the ductility begins to degrade in the drawn wire at about 50% working rate in the case of a drawing speed of 50 m/min, it is difficult to obtain a highly ductile drawn wire with a strength greater than 200 kgf/mm.sup.2.
Further, steel materials represented by steel No. A and having the chemical compositions shown in Table 1 are rolled into wire rods, followed by direct hardening to obtain a wire rod of 5.5 mm diameter having a structure mainly composed or martensite, which are re-heated into a ferrite-austenite 2-phase region followed by water cooling to obtain an intensely workable wire rod having a mixed structure, in which fine acicular martensite is uniformly dispersed by 21% volume ratio into the ferrite phase. Then the wire rod is drawn at a low speed or drawn at a speed of 30-530 m/min. As shown by the result in FIG. 2, a high strength drawn wire having a tensile strength greater than 320 kgf/mm.sup.2 can be obtained at 99.9% working rate in the case of a drawing speed of 1 m/min, but it is difficult to obtain a drawn wire having a tensile strength greater than 200 kgf/mm.sup.2 in the case of the continuous drawing at a speed or 30-530 m/min since the ductility begins to degrade from a working rate of about 95%.